The European Council has adopted and published its own Rules of Procedure.
The Rules of Procedure deal with such matters as the preparation and follow-up to the proceedings of the European Council, the preparation of its meetings and its agenda, its composition, the composition of national delegations, representation before the European Parliament, the quorum and the language regime.
Rather sadly, the Rules specify that the meetings of the European Council will be held in secret. It limits the number of persons who can make up national delegations to 20. That's a circus of 540 people, plus 20 for the Commission and five for the High Representative of the Union for Foreign Affairs and Security Policy.